This invention relates to a rotation transmitting device used e.g. for changing over the mode of transmission of driving force on a four-wheel drive car.
Four-wheel drive cars intended for off-the-road travel are equipped with a secondary transmission changing over the driving mode. Such a driving system is called a part-time drive system which allows the driver to change over the drive mode between the four-wheel drive and the two-wheel drive according to the road condition. Since such a four-wheel drive car is required to transmit a large driving force while travelling at low speed, it is driven with the four wheels mechanically and directly coupled together.
However, the structure in which the four wheels are mechanically and directly coupled together has a problem in that when the car is turning a tight corner in urban areas, slip occurs between the front wheels which tend to turn Faster and the Fear wheels which tend to turn slower due to a difference in the turning radii of the front and Fear wheels. As a result, the car behaves as though it were braked. On the other hand, a four-wheel drive car equipped with only a center differential has a problem in that if one of the wheels should lose grip of the road surface and slip, the driving force is transmitted only to this wheel while no driving force is transmitted to the other wheels gripping the road surface.
To cope with this problem, center differentials are under development today which have both the differential Function of absorbing any rotational difference between the Front and rear wheels and the function of limiting the differential function in case one of the wheels slips. Typical examples of such differentials are a viscous coupling which utilizes the shearing resistance of highly viscous substance and a coupling which utilizes the frictional force between the multiple plate clutch and a resilient member.
Unfortunately, those couplings represented as viscous couplings have a problem in that no sufficient torque transmission is possible and thus no sufficient driving force is obtainable in a low-speed region, for example while the car is running off the road, at which time the car speed is generally low, because these couplings utilize a speed difference between the input side and the output side to transmit torque.
In order to increase the torque transmitted in a low-speed region, the coupling capacity has to be increased because these couplings utilize viscous resistance or Frictional force in transmitting torque. This makes the coupling bulky. On the other hand, the use of fluid having a higher viscosity to increase the torque transmitted would cause a large dragging torque when turning at a low speed.
Moreover, the use of such a complicated center differential will result in a more complicated and heavier and thus more costly driving system.